1. Field of the Invention
The invention relates to a steering system.
2. Description of the Related Art
Steering systems are available which enable involvement in control of steering of a steering wheel and turning of steered wheels by allowing a motor to generate a torque to assist a steering mechanism that turns the steered wheels based on the steering of the steering wheel. In some of such steering systems, the involvement in control is enabled so as to mitigate impact when a so-called end abutment occurs, in which a steering angle of the steering wheel reaches a maximum steering angle, i.e., steering end (for example, Japanese Patent No. 5556219 (JP 5556219 B)).
In JP 5556219 B, the impact of end abutment is controllably mitigated by setting a limiting angular velocity for a steering angular velocity of steering generated on the steering wheel when the steering angle is close to the maximum steering angle. Specifically, the limiting angular velocity is calculated so that the limiting angular velocity decreases with an increase in the steering angle when the steering angle is equal to or larger than a predetermined steering angle close to the maximum steering angle. In JP 5556219 B, a compensation amount proportional to the difference between the limiting angular velocity and the steering angular velocity is calculated to compensate for (correct) a voltage command value.
In JP 5556219 B, the voltage command value is compensated for based on an input of the steering angular velocity. However, the voltage command value itself, which is to be compensated for, may increase or decrease as a result of a fluctuation in the voltage of a power supply (for vehicles, the power supply voltage of a battery). Thus, when the compensation for the voltage command value is implemented under proportional control in view of following capability of the motor to the voltage command value, the following situation may occur. For example, when the power supply voltage rises to a value larger than an initially designed and estimated value to increase the voltage command value itself, which is to be compensated for, even with the calculation for a reduction in the limiting angular velocity, the increase in the steering angular velocity corresponding to the increase in the voltage command value remains as a steady-state deviation. On the other hand, when the limiting angular velocity is designed to be low, in view of such influence of the power supply voltage, at the initial design and estimation stage, the range of adjustment of the limiting angular velocity is narrowed, reducing the degree of freedom of the design.